1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a printer, that uses electrophotography and electrostatic recording.
2. Related Background Art
FIG. 2 is a sectional view of a digital copying machine using electrophotography.
In this image forming apparatus shown in FIG. 2, when a copy start signal is input a charger 3 evenly charges the surface of a photosensitive drum 1 to a predetermined potential. A unit 9 is an original reader which is an integrated unit of an original lamp, a short-focus lens array, and a CCD sensor. When this unit 9 scans the surface of an original G placed on an original table 8 by irradiating its surface with light, the short-focus lens array of the unit 9 forms an image of the scanning light illuminating the original surface and reflected by it and feeds this image to the CCD sensor.
The CCD sensor includes a light-receiving unit, a transfer unit, and an output unit. The CCD light-receiving unit converts light reflected by the original surface and fed as described above as an optical signal, into an electric charge signal. The transfer unit sequentially transfers this electric charge signal to the output unit in synchronism with clock pulses. The output unit converts the electric charge signal into a voltage signal and outputs the signal after amplifying it and lowering its impedance. In this way, an analog signal fed as light reflected by the original surface is converted into a digital signal by the well-known processing, and this digital image signal is transferred to a printer unit. In this printer unit, a laser exposing means 2 scans light from a solid-state laser element, which turns on and off its emission upon reception of the image signal, by using a rotational polygonal mirror that rotates at high speed. This laser exposing means 2 forms an electrostatic latent image corresponding to the original image on the surface of the photosensitive drum 1 that is evenly charged.
A developing device 4 containing a so-called two-component developer mix having toner particles and carrier particles develops the electrostatic latent image formed on the surface of the photosensitive drum 1, thereby forming a toner image on the photosensitive drum 1. A transfer device 7 electrostatically transfers the toner image thus formed on the photosensitive drum 1 onto a transfer material. After that, the transfer material having the transferred toner image is electrostatically separated and conveyed to a fixing device 6, thermally fixed by this fixing device 6, and output as a fixed image.
In the image forming apparatus shown in FIG. 2, during the aforementioned image formation a cleaner 5 removes adhered contaminants, such as untransferred toner, from the surface of the photosensitive drum 1 after the toner image is transferred onto the transfer material, so that the photosensitive drum 1 can be used in repetitive image formation. This untransferred toner collected by the cleaner 5 is usually discarded as waste toner.
It is, however, preferable not to produce such waste toner in respect of environmental protection. In recent years, therefore, a so-called cleaner-less apparatus has appeared from which the cleaner 5 is removed and which cleans as it develops by the developing device 4. In this method, toner slightly remaining on the photosensitive drum 1 after a toner image is transferred is collected by a defogging potential difference V.sub.back which is the potential difference between the DC voltage applied to the developing device 4 and the surface potential of the photosensitive drum 1. This method can eliminate waste toner because untransferred toner is collected and used in the subsequent steps. Also, since no cleaner need to be used, the method is advantageous in respect of the internal space of the apparatus, so the apparatus can be greatly miniaturized.
Recently, a contact charging device, i.e., a device which charges an object to be charged (photosensitive drum) by bringing a voltage-applied charging member in contact with the object, has been put into practical use as the abovementioned charger for evenly charging the surface of the photosensitive drum 1. This is so because this device has the advantages of low ozone generation and low consumption power.
As the charging member used in the charging device of this type, a magnetic brush is preferably used because of the stability of charging contact. In a magnetic brush type contact charging device, conductive magnetic particles are magnetically constrained directly on a magnet, or on a sleeve containing a magnet, thereby forming a magnetic brush. This magnetic brush, as it is at rest or rotated, is brought into contact with an object to be charged and is applied with voltage to start charging.
Especially when such a contact charging device is used and an object to be charged is either a photosensitive body having a surface layer, in which fine conductive particles are dispersed, on a common organic photosensitive body or an amorphous silicon photosensitive body, a charging potential substantially equal to the DC component of the bias applied to the contact charging member can be obtained on the surface of the object to be charged. This method of charging is particularly called injection charging. When this injection charging method is used, an object to be charged is charged without using any discharge phenomenon, unlike in a method which usually uses a corona charger. Accordingly, completely ozoneless, low-power-consumption charging is possible, so the method has attracted attention recently.
Japanese Laid-Open Patent Application No. 9-096997 describes one example of the method which cleans as it develops by using this magnetic brush type contact charging device. The magnetic brush type contact charging device using magnetic particles is more advantageous to contamination by toner than a device using a conductive roller or the like. Also, this device is so constructed as to temporarily collect untransferred toner by magnetic particles and return the toner to a photosensitive body. Therefore, untransferred patterns are uniformly spurted and do not form patterns. This reduces the influence on image exposure.
Unfortunately, in an image forming apparatus using this contact charging system, if image formation is repetitively performed for long time periods while a developing means collects residual toner simultaneously with development without using any cleaner means, "positive ghost" occurs which is a phenomenon in which thin previous images remain because the developing means becomes unable to sufficiently collect untransferred toner. More specifically, this "positive ghost" occurs because portions of the photosensitive drum 1 below untransferred toner cannot be evenly charged when the contact charging member passes by. This phenomenon becomes conspicuous when the contact charging member is contaminated with toner.
This contamination of the contact charging member can be removed more advantageously when the contact charging member contains magnetic particles, than when a conductive roller or a conductive brush is used. If this is the case, however, toner particles having relatively high electrical resistance are commonly used as a developer mix. Therefore, when the apparatus is used for long time periods, the resin component of the toner particles becomes fused to the magnetic particles of the contact charging member by which the toner is temporarily collected, or additives added to the surfaces of these toner particles adhere to the magnetic particles. Consequently, contamination and the like of the conductive sleeve surface carrying the magnetic particles occur more significantly.
One cause of this contamination is toner having high electric charge. That is, toner having high electric charge is not easily discharged from the charging member onto the photosensitive body, owing to the influence of the magnetic mirror force, when temporarily collected by the charging member. If the friction between the magnetic particles, or between the magnetic particles and the magnetic particle carrier, is repeated in this state, fusion to the contact charging member is brought about. These phenomena raise the resistance of the whole or a part of the contact charging member. As a consequence, the surface of the photosensitive drum as an object to be charged can no longer be charged to a desired potential, or uneven charging takes place. The results are positive ghost as described above and some other image defects.